Electronic equipment is generally highly reliable when operated within environmental and electrical design specifications but is vulnerable to failure when such specifications are exceeded. Electronic equipment is particularly vulnerable to exposure to excessively high voltages, even if exposure is extremely brief, such as when a transient spike is injected into a power supply line. Such transients may only last for microseconds but may expose the connected circuitry to extremely high voltages several orders of magnitude greater than the nominal supply voltage. Where the load on a supply line is very large and distributed, such as is the case with distributed supplies in a residential or industrial area, supply authorities protect supplies with air-gap surge protectors which remove the worst of transients from the supply network, such that any remaining transients may be absorbed by the distributed load without significant damage. However consumers are often urged to protect sensitive equipment such as computers and expensive audio/visual equipment from residual surges with commercial power supply filters.
On the other hand, where equipment is connected to a dedicated supply line and particularly where the equipment is at well-spaced locations on a linear supply line such as in a railway signalling system, the equipment may be exposed to significant transients due to lightning strike. In the case of railway systems a further problem may exist where passing trains also inject significant noise levels into supply lines.
Railways routinely use Surge Reduction Filters (SRFs) to locally protect the Signalling power system(s) from potentially damaging power fluctuations as a result of supply-side lightning strikes, power surges or other undefined transient currents. Other important infrastructure having sensitive equipment installations such as telephone exchanges and mobile phone transmitter towers, hospitals, computer server farms, airport control towers and military installations may also use SRFs to minimise outages due to supply fluctuations.
In the past, SRFs have incorporated protection elements (typically metal-oxide varistors (MOVs)) which are semi-permanently mounted using bolted connections, thus requiring disassembly of the circuit, or swapping out of the entire SRF, when the protection element becomes unserviceable. In the case of railway signalling systems, the frequency of replacement for the protection element in some areas may be as often as every 6 months. Maintenance is therefore time consuming and requires the supply to be powered down while maintenance is performed, which for a railway system means halting of trains. A rail network may have hundreds of SRFs distributed over a vast area.
Another issue for railway systems is that of vibration. Trackside installations exist in a high vibration environment with operators often requiring that equipment be rated to withstand vibration forces of at least 11 G.
High resistance connections can form due to loosening of terminal connections due to vibration. Where bolted connections include multiple connections on a single stud, loosening of one connection may also occur when another connection is tightened, inadvertently causing a high resistance contact in the loosened connection, leading to a fault.
SRFs associated with remotely located equipment may also fail without the knowledge of operators, until such time as the equipment being protected malfunctions causing a serious safety hazard.